An ink-jet recording apparatus such as an ink-jet printer or an ink-jet plotter moves a recording head in a main scanning direction and moves a recording paper (a kind of recording medium) in a subordinate scanning direction. Cooperating with the movement, drops of ink are jetted from nozzles of the recording head, so that an image (or a character) can be recorded. The drops of the ink are jetted for example by causing pressure generating chambers communicating with the nozzles to expand and contract.
The ink in the nozzles of the recording head is exposed to air. Thus, solvent of the ink such as water may gradually evaporate to increase a viscosity of the ink in the nozzles. In the case, quality of printed (recorded) images may deteriorate because the ink having a great viscosity may be jetted toward a direction deviated from a normal direction.
To prevent the viscosity of the ink in the nozzles from increasing, some measures have been proposed. One of the measures is to cause a meniscus of the ink to minutely vibrate to stir the ink. The meniscus means a free surface of the ink exposed at an opening of the nozzle.
For stirring the ink, the meniscus may be vibrated to a jetting direction of the ink and to a contracting direction opposed to the jetting direction by turns in such a manner that the ink may not be jetted. The vibration of the meniscus can be also carried out by expanding and contracting of the pressure chambers. Owing to the vibration of the meniscus, the ink at the opening of the nozzle may be stirred to prevent the viscosity of the ink from increasing.
The stirring of the ink may be carried out during a recording operation. For example, the stirring may be carried out while a carriage carrying the recording head is being accelerated after starting a main scanning, or while a recording (printing) operation for a line is being carried out. In the stirring while the carriage is being accelerated, a micro-vibrating operating signal for micro vibrating is supplied to the recording head to cause all menisci in the nozzles to minutely vibrate. In the stirring while the recording operation is being carried out, a pulse signal for micro vibrating is generated from a jetting operating signal for jetting ink, and the pulse signal is supplied to the recording head. Thus, the ink in the nozzles not in the recording (jetting) operation may be stirred.
The pressure generating chambers may be caused to expand and contract for example by causing piezoelectric members provided in contact with walls defining the pressure generating chambers to extend and contract.
In addition, Japanese Patent Laid-Open Publication No.2000-21507 has described that it is effective to cause menisci of ink in nozzles to minutely vibrate during a predetermined time from a suitable timing just before jetting a drop of the ink, or from a suitable timing just before jetting a drop of the ink till another suitable timing just before jetting a drop of the ink.
When the menisci are caused to vibrate by means of the piezoelectric members or the like, noise may be generated from the piezoelectric members or the like, which may become a problem. Especially, when amplitude of the vibration is increased to enhance the effect of the micro-vibrating operation, the noise also may be increased. Herein, it is said that a hearable frequency band of a human being is 20 to 20000 Hz, as shown in FIG. 19. Especially, a human being can hardly hear noise whose frequency is not less than 15 kHz. Thus, it is preferable that the piezoelectric members or the like is operated at a frequency not less than 15 kHz.
However, when the ink includes a solid component such as pigment, if the frequency of the micro-vibration is raised too much, the nozzles may drip with the ink, so that the ink may not be jetted from the nozzles accurately but deflected. In general, a suitable micro-vibrating frequency depends on natural frequency TM of the nozzles.
In addition, in general, the micro-vibrating operating signal is formed as a signal wherein the same pulse wave repeatedly appears at a predetermined frequency. If the frequency is not high enough, the effect of recovering viscosity of the ink from an increased state thereof may not be sufficiently achieved.
Through carrying out various experiments regarding the effect of recovering the viscosity of the ink from an increased state thereof, the inventor has found that: if the above frequency is not less than 10.8 kHz, deflection of firstly jetted drop of the ink can be substantially completely prevented.
To the contrary, if the frequency is too high, the nozzle may drip with the ink, so that the ink may not be jetted from the nozzle accurately but deflected.
Through carrying out various experiments regarding generation of the dripping with the ink at the nozzle, the inventor has found that: if the above frequency is not more than 25.0 kHz, the dripping with the ink at the nozzle can be substantially completely prevented.
That is, in order to recover the viscosity of the ink from an increased state thereof and to prevent generation of the dripping with the ink at the nozzle, it is sufficient that the above frequency is not less than 10.8 kHz and not more than 25.0 kHz.
Herein, if an actuator (micro-vibrating unit) for causing the pressure generating chambers to expand and contract in order to carry out a micro-vibrating operation consists of a PZT device (piezoelectric member), the above frequency corresponds to a driving frequency of the PZT device. In general, when a PZT device is driven at a frequency, noise of the frequency is generated.
It is said that a hearable frequency region for a human ear is 20 Hz to 20 kHz. Among the region, it is said that a barbarous frequency region is 1 kHz to 16 kHz. That is, for the human ear, a sound having a frequency not more than 1 kHz and a sound having a frequency not less than 16 kHz are not barbarous.
That is, in view of suppressing generation of the noise from the ink-jetting apparatus, it is sufficient that the above frequency is not more than 1 kHz or not less than 16 kHz.
Thus, a suitable range for the above frequency is not less than 16 kHz and not more than 25 kHz, with respect to all the above points, that is, recovering the viscosity of the ink from an increased state thereof, preventing generation of the dripping with the ink at the nozzle, and suppressing generation of the noise.
In the suitable range, the inventor planed to adopt a frequency of 17.27 kHz as a standard specification for a micro-vibrating signal.
However, the inventor has found that: if the frequency of 17.27 kHz is set, the following problem may arise.
For example, if the micro-vibrating unit is formed by a PZT device as described above, an electric circuit for driving the PZT device is necessary in general. In the electric circuit, transistors are used.
However, a driving frequency of 17.27 kHz may cause the transistors in the electric circuit to generate great heat, which may cause various problems.
In order to solve the problem regarding the heat generation of the transistors, some measures maybe proposed, such as arranging a large heat sink or arranging a fan. However, these measures may raise costs.
As shown in table 1, the inventor has found that: it is preferable to lower the driving frequency of the PZT device to 13 kHz or less, in view of forming the electric circuit.
TABLE 1Micro-Vibrating FrequencyTransistor-Heat-GenerationJudgement17.27 kHz163.3° C.x16.0  kHz154.4° C.x13.0  kHz133.6° C.∘
Integrating the above aspects by the inventor, while a suitable range for the above frequency is not less than 16 kHz and not more than 25 kHz with respect to the three points of: recovering the viscosity of the ink from an increased state thereof, preventing generation of the dripping with the ink at the nozzle, and suppressing generation of the noise, a suitable range for the above frequency is not more than 13 kHz with respect to easiness of forming a driving circuit for the micro-vibrating unit. Thus, there is no frequency region satisfying the above both ranges.